Light emitting diodes (“LEDs”) are compact inexpensive solid-state devices which produce light when a suitable electric current is applied. They are extensively used as display indicators for electronic and computing devices. They are also widely used in large arrays to display graphics and text information in raster form.
Operating an array of LEDs to display graphical data is typically performed by connecting them in a passive-matrix configuration and driving them column by column in a time-multiplexed manner. During each column-scan period, row drivers are selectively enabled or disabled, causing the LEDs at those row/column intersections to emit light, in accordance with the desired image output. This column-scan process occurs rapidly enough, so that the individual LED emission pulses appear continuous, and the image is coherent. This arrangement and method reduces complexity and drive electronics enough to make arrays of a useful size at all practical.
Most LEDs can also operate as photodiodes, or light detectors, as well. Though they typically designed for only their emitting, and not their detecting role, most can function effectively in either role.
The use of LEDs in a bidirectional manner is known, e.g., in LED copier/scanner heads, as described in U.S. Pat. No. 4,424,524 to Daniele, and U.S. Pat. No. 5,424,855 to Nakamura.
Furthermore, it is possible to utilize the same set of LEDs for both display and sensing purposes simultaneously, by rapidly multiplexing these functions in time. This is possible without sacrificing the display capabilities, if the sensing is done fast enough to be beyond the scope of human perception.
U.S. Pat. No. 4,692,739 to Dorn describes the use of LEDs in this manner to create a touch sensing display. The features described in this publication include an LED array that has been configured for simultaneously emitting and detecting light, and registers touches by detecting the attenuation of background illumination when a finger covers one or more LEDs. However, this occlusion-based approach may be prone to false triggering by shadows. It also may require the presence of a stable external illumination source. Finally it does not apply to passive-matrix configurations, and hence is not scalable in practice to large arrays.
Touch sensing mechanisms have also been described that operate by employing active emitter/detector pairs, and sensing fingers and objects through the reflection of light, as described in U.S. Pat. No. 3,621,268 to Friedrich. However, this approach likely needs the addition of a large number of dedicated photo-detectors, resulting in increased cost and complexity in component layout and in wiring. Furthermore, such arrangement may still be prone to false triggers due to external illumination.